Ionization Shifts in 1 H‐NMR Spectra of α,β‐Unsaturated Carboxylic Acids

Changes in chemical shifts of olefinic protons in a number of α,β‐ and α,β,γ,δ‐unsaturated carboxylic acids caused by ionization of the COOH group were investigated. The ionization shifts of α‐H‐atoms are −0.09 to 0.07 ppm, those of β‐H‐atoms are 0.32−0.47 ppm. The ionization shifts of δ‐H‐atoms are substantially larger than those of γ‐H‐atoms. The ionization shifts can be used for immediate determination of the esterification site in monoesters of (2 E ,4 Z )‐2,4‐hexadienedioic (muconic) acid, which are of interest in connection with synthetic studies on verrucarins. Thus, isomerization by heating in aqueous solution of monoesters of (2 Z ,4 Z )‐2,4‐hexadienedioic acid yields 1‐monoesters rather than 6‐monoesters of (2 E ,4 Z )‐2,4‐hexadienedioic acid, in accordance with the isomerization mechanism involving anchimeric assistance of the free COOH group. Solutions of the ABXY spectra of olefinic protons of monomethyl (2 E ,4 E )‐ and (2 Z ,4 Z )‐2,4‐hexadienedioate are reported.